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Date: Thu, 14 Jul 2022 11:16:45 +0800
From: Schspa Shi <schspa@...il.com>
To: jiangshanlai@...il.com, tj@...nel.org
Cc: linux-kernel@...r.kernel.org, zhaohui.shi@...izon.ai,
Schspa Shi <schspa@...il.com>
Subject: [PATCH v3] workqueue: Use active mask for new worker when pool is DISASSOCIATED
When CPU-[un]hotplugs, all workers will be bound to active CPU via
unbind_workers().
But the unbound worker still has a chance to create a new worker, which
has bound the newly created task to pool->attrs->cpumask. But the CPU has
been unplugged.
Please refer to the following scenarios.
CPU0 CPU1
------------------------------------------------------------------
sched_cpu_deactivate(cpu_active_mask clear)
workqueue_offline_cpu(work pool POOL_DISASSOCIATED)
-- all worker will migrate to another cpu --
worker_thread
-- will create new worker if
pool->worklist is not empty
create_worker()
-- new kworker will bound to CPU0
(pool->attrs->cpumask will be mask of CPU0).
kworker/0:x will running on rq
sched_cpu_dying
if (rq->nr_running != 1 || rq_has_pinned_tasks(rq))
WARN(true, "Dying CPU not properly vacated!");
---------OOPS-------------
And the crash log as fellowing.
The crash log is as follows:
[ 1622.829074] ------------[ cut here ]------------
[ 1622.829081] Dying CPU not properly vacated!
[ 1622.829091] WARNING: CPU: 3 PID: 31 at kernel/sched/core.c:7756 sched_cpu_dying+0x74/0x204
[ 1622.829374] CPU: 3 PID: 31 Comm: migration/3 Tainted: P O 5.10.59-rt52 #2
[ 1622.829386] Stopper: multi_cpu_stop+0x0/0x160 <- 0x0
[ 1622.829400] pstate: 60c00009 (nZCv daif +PAN +UAO -TCO BTYPE=--)
[ 1622.829408] pc : sched_cpu_dying+0x74/0x204
[ 1622.829415] lr : sched_cpu_dying+0x74/0x204
[ 1622.829421] sp : ffff800012933c70
[ 1622.829424] pmr_save: 00000060
[ 1622.829426] x29: ffff800012933c70 x28: 0000000000000000
[ 1622.829435] x27: 0000000000000000 x26: 0000000000000001
[ 1622.829444] x25: 0000000000000000 x24: ffff800018353c2c
[ 1622.829452] x23: 0000000000000003 x22: 0000000000000003
[ 1622.829460] x21: 0000000000000059 x20: 0000000000000000
[ 1622.829468] x19: ffff00027ee68a80 x18: 0000000000000000
[ 1622.829477] x17: 0000000000000000 x16: 0000000000000000
[ 1622.829485] x15: ffffffffffffffff x14: ffff80001169ae30
[ 1622.829493] x13: ffffffffffc38c07 x12: ffffffffffffffff
[ 1622.829501] x11: ffffffffffe00000 x10: ffff80001169ae58
[ 1622.829510] x9 : 000000000000001e x8 : ffff80001169ae30
[ 1622.829518] x7 : ffff800012933ab0 x6 : 00000000ffff0e20
[ 1622.829526] x5 : ffff00027ee62988 x4 : 00000000ffff0e20
[ 1622.829535] x3 : ffff800011e30180 x2 : 0000000100000002
[ 1622.829543] x1 : 0000000000000000 x0 : 0000000000000000
[ 1622.829552] Call trace:
[ 1622.829555] sched_cpu_dying+0x74/0x204
[ 1622.829562] cpuhp_invoke_callback+0xc0/0x1b0
[ 1622.829571] take_cpu_down+0xbc/0xd4
[ 1622.829577] multi_cpu_stop+0x138/0x160
[ 1622.829584] cpu_stopper_thread+0x9c/0x118
[ 1622.829591] smpboot_thread_fn+0x1e8/0x1ec
[ 1622.829598] kthread+0x114/0x124
[ 1622.829606] ret_from_fork+0x10/0x30
[ 1622.829615] ---[ end trace 0000000000000002 ]---
[ 1623.830273] CPU3 enqueued tasks (2 total):
[ 1623.830291] pid: 31, name: migration/3
[ 1623.830440] pid: 25654, name: kworker/3:0
[ 1623.830444] task:kworker/3:0 state:R running task stack: 0 pid:25654 ppid: 2 flags:0x00000028
[ 1623.830458] Call trace:
[ 1623.830460] __switch_to+0x164/0x17c
[ 1623.830472] __schedule+0x4cc/0x5c0
[ 1623.830483] schedule+0x7c/0xcc
[ 1623.830491] schedule_preempt_disabled+0x14/0x24
[ 1623.830500] kthread+0xd8/0x124
[ 1623.830509] ret_from_fork+0x10/0x30
In the crash log, the error enqueued kworker(pid: 25654, name: kworker/3:0)
is a newly created thread, and have bind to a unpluged CPU 3.
Can crash dump can also verify this.
crash> task -R nr_cpus_allowed,cpus_mask 25654
PID: 25654 TASK: ffff000181ff0000 CPU: 3 COMMAND: "kworker/3:0"
nr_cpus_allowed = 1,
cpus_mask = {
bits = {8, 0}
},
crash> struct worker_pool.cpu,nr_workers,attrs 0xffff00027ee68380
cpu = 3,
nr_workers = 3,
attrs = 0xffff000180004480,
crash> struct workqueue_attrs 0xffff000180004480
struct workqueue_attrs {
nice = 0,
cpumask = {{
bits = {8, 0}
}},
no_numa = false
}
The stack trace of the bad running task was dumped via the following patch:
Link: https://lore.kernel.org/all/20220519161125.41144-1-schspa@gmail.com/
And I think this debug patch needs to be added to the mainline,
it can help us to debug this kind of problem
To fix it, we can use cpu_active_mask when work pool is DISASSOCIATED.
Signed-off-by: Schspa Shi <schspa@...il.com>
--
Changelog:
v1 -> v2:
- Move worker task bind to worker_attach_to_pool, remove extra
wq_pool_attach_mutex added.
- Add a timing diagram to make this question clearer.
v2 -> v3:
- Add missing PF_NO_SETAFFINITY, use cpumask_intersects to
avoid setting bad mask for unbound work pool as Lai Jiangshan
advised.
- Call kthread_set_pre_cpu correctly for unbound worker.
---
kernel/workqueue.c | 16 +++++++++++-----
1 file changed, 11 insertions(+), 5 deletions(-)
diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 1ea50f6be843..b3e9289d9640 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1860,8 +1860,16 @@ static struct worker *alloc_worker(int node)
static void worker_attach_to_pool(struct worker *worker,
struct worker_pool *pool)
{
+ const struct cpumask *cpu_mask;
+
mutex_lock(&wq_pool_attach_mutex);
+ if (cpumask_intersects(pool->attrs->cpumask, cpu_active_mask))
+ cpu_mask = pool->attrs->cpumask;
+ else
+ cpu_mask = wq_unbound_cpumask;
+
+ set_cpus_allowed_ptr(worker->task, cpu_mask);
/*
* The wq_pool_attach_mutex ensures %POOL_DISASSOCIATED remains
* stable across this function. See the comments above the flag
@@ -1870,10 +1878,8 @@ static void worker_attach_to_pool(struct worker *worker,
if (pool->flags & POOL_DISASSOCIATED)
worker->flags |= WORKER_UNBOUND;
else
- kthread_set_per_cpu(worker->task, pool->cpu);
-
- if (worker->rescue_wq)
- set_cpus_allowed_ptr(worker->task, pool->attrs->cpumask);
+ kthread_set_per_cpu(worker->task,
+ cpu_mask == wq_unbound_cpumask ? -1 : pool->cpu);
list_add_tail(&worker->node, &pool->workers);
worker->pool = pool;
@@ -1952,8 +1958,8 @@ static struct worker *create_worker(struct worker_pool *pool)
goto fail;
set_user_nice(worker->task, pool->attrs->nice);
- kthread_bind_mask(worker->task, pool->attrs->cpumask);
+ worker->task->flags |= PF_NO_SETAFFINITY;
/* successful, attach the worker to the pool */
worker_attach_to_pool(worker, pool);
--
2.29.0
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